thunk.js

/**
 * Thunk function returns a function that takes all args except the last `cb` that will return `err` or `data
 * @param {Function} fn the function to wrap. ex. fs.readFile
 * @param {Object|undefined} ctx the context to call the original `fn` with
 * @return {Function} first function to be called with all args except the `cb`
 * ex. const read = thunk(fs.readFile);
 *     const stuffFromTheCb = yield read('/path/to/cool/stuff.js');
 * 
 * ex. const cp = child_process.spawn('./path/to/some/binary', ['--cool', 'stuff']);
 *     const cpEventThunked = thunk(cp.on, cp); //pass the child process as context or it breaks
 *     const closedCode = yield cpEventThunked('close');
 * 
 * function returned doesn't take any arguments
 * but we may want to do stuff after the async action happens
 * ex. const startTunnel = thunk(browserStackTunnel.start, browserStackTunnel); 
 *     try {                                                                    
 *       yield startTunnel();                                                   
 *     } catch (err) {
 *       //do "whatevs" with the error
 *     }
 */
export default function(fn, ctx) {
  //all arguments except the `cb`, maybe there will be 0 arguments
  return (...args) => {
    //if there are no args then just call the original function with the cb
    return (cb) => {
      fn.apply(ctx || null, args.length ? [...args, cb] : [cb]);
    };
  };
}